Alzheimer's disease (AD) is the most common cause of dementia in later life. Pathologically AD is characterized by the deposition in the brain of amyloid in extracellular plaques and intracellular neurofibrillary tangles. The amyloid plaques are mainly composed of amyloid peptides (Abeta peptides) which originate from the β-Amyloid Precursor Protein (APP) by a series of proteolytic cleavage steps. Several forms of APP have been identified of which the most abundant are proteins of 695, 751 and 770 amino acids length. They all arise from a single gene through differential splicing. The Abeta peptides are derived from the same domain of the APP but differ at their N- and C-termini, the main species are of 40 and 42 amino-acid length.
Abeta peptides are produced from APP through the sequential action of 2 proteolytic enzymes termed β- and γ-secretase. β-Secretase cleaves first in the extracellular domain of APP just outside of the trans-membrane domain (TM) to produce a C-terminal fragment of APP containing the TM- and cytoplasmatic domain (CTFβ). CTFβ is the substrate for γ-secretase which cleaves at several adjacent positions within the TM to produce the Aβ peptides and the cytoplasmic fragment. The majority of Abeta peptides is of 40 amino acids length (Aβ40), a minor species carries 2 additional amino acids at its C-terminus. The latter is supposed to be the more pathogenic amyloid peptide.
The β-secretase is a typical aspartyl protease. The γ-secretase is a proteolytic activity consisting of several proteins, its exact composition is incompletely understood. However, the presenilins are essential components of this activity and may represent a new group of atypical aspartyl proteases which cleave within the TM of their substrates and which are themselves polytopic membrane proteins. Other essential components of γ-secretase may be presenilin, nicastrin and the products of the aph1 and pen-2 genes. Proven substrates for γ-secretase are the APP and the proteins of the Notch receptor family, however, γ-secretase has a loose substrate specificity and may cleave further membrane proteins unrelated to APP and Notch. It was demonstrated by genetic means, i.e., ablation of either the presenilin 1 and 2 genes or the nicastrin gene, that γ-secretase is absolutely required for Notch signaling. This was subsequently confirmed by treatment with specific γ-secretase inhibitors.
Notch receptors are not only essential in embryonal development but also play a critical role in several tissues of the adult organism which continue to undergo proliferation and differentiation, e.g., hematopoietic cells and epithelia of the gut and skin. The signaling of Notch receptors occurs through an ordered sequence of events: binding to a ligand of the Delta or Jagged group, cleavage of the extracellular domain by an ADAM protease (TACE) and subsequent cleavage by the γ-secretase within the Notch transmembrane domain. The latter cleavage results in the liberation of the cytoplasmic domain which then translocates to the nucleus where it acts with other proteins as a regulator of a specific group of genes.
A role for Notch in human oncogenesis was most clearly established for T-cell Acute Lymphoblastic Leukemia (T-ALL). Some rare cases of T-ALL show a (7:9) chromosomal translocation which leads to a constitutive activation of Notch1. Recently it was reported that ca. 50% of all T-ALL cases have point mutation in the Notch1 receptor which also cause over-activation. It was shown that growth of some cell lines derived from such leukemias were sensitive to treatment with γ-secretase inhibitors which confirmed an essential role for Notch1 signaling.
A broader role for Notch in oncogenesis is discussed in several recent paper which describe that its signaling is required for maintaining the neoplastic phenotype in ras-transformed cells. Deregulation of the ras-signaling pathway is found in a number of common cancers including cervical carcinomas and breast carcinomas.
The γ-secretase activity is absolutely required for the production of Abeta peptides. This has been shown both by genetic means, i.e., ablation of the presenilin genes and by low-molecular-weight inhibitory compounds. Since according to the amyloid hypothesis of AD the production and deposition of Abeta is the ultimate cause for the disease, it is thought that selective and potent inhibitors of γ-secretase will be useful for the prevention and treatment of AD.
Numerous documents describe the current knowledge on γ-secretase inhibition, for example the following publications:    The EMBO Journal (2204), 23, 483-488,    Biochemistry (2004), 43 (30), 9774-9789,    Nature Reviews/Neuroscience, Vol. 3, April 2002/281,    Biochemical Society Transactions (2002), Vol. 30. part 4,    Current Topics in Medicinal Chemistry, 2002, 2, 371-383,    Current Medicinal Chemistry, 2002, Vol. 9, No. 11, 1087-1106,    Drug Development Research, 56, 211-227, 2002,    Drug Discovery Today, Vol. 6, No. 9, May 2001, 459-462,    FEBS Letters, 483, (2000), 6-10,    Science, Vol. 297, 353-356, July 2002,    Journ. of Medicinal Chemistry, Vol. 44, No. 13, 2001, 2039-2060,    Nature Cell Biology 2, 461-462, 2000,    Nature 398, 518-522, 1999,    Nature Cell Biology 3, 1129-1132, 2001,    PNAS 98, 7487-7491, 2001,    Cancer Cell 1, 75-87, 2002,    Science 306, 269-271, 2004,    Mol Cell Biol 23, 655-664, 2003,    Nature Medicine 8, 979-986, 2002 and    Oncogene 22, 6598-6608, 2003.